An image forming apparatus such as an electronic copying machine, facsimile, or printer forms an electrostatic latent image on the surface of a photosensitive drum serving as an image carrier by scanning that surface. The apparatus visualizes the electrostatic latent image with a developer mix (toner), and transfers the image onto a sheet.
A laser beam emitted by a laser diode strikes a galvanomirror, and is reflected by it to strike a polygon mirror. The laser beam having struck the polygon mirror strikes the polygon mirror, and is reflected by it to scan the surface of the photosensitive drum axially along the photosensitive drum as the polygon mirror pivots. Axial scanning of the photosensitive drum is called main scanning. This main scanning is repetitively executed with rotation of the photosensitive drum. The moving direction (direction perpendicular to the main scanning direction) of each main scanning repeated on the photosensitive drum is called a subscanning direction.
The laser beam position in the subscanning direction is important in forming a high-quality image, and can be adjusted by the galvanomirror. However, the laser beam position in the sub-scanning direction, which has been adjusted once, may vary owing to vibrations and the like.